{{Warning|Systemd is an asynchronous starting process. If you bind the SSH daemon to a specific IP address {{ic|ListenAddress 192.168.1.100}} it may fail to load during boot since the default sshd.service unit file has no dependency on network interfaces being enabled. When binding to an IP address, you will need to add {{ic|After&#61;network.target}} to a custom sshd.service unit file. See [[Systemd#Replacing provided unit files]].}}

−

Or you can enable SSH Daemon socket so the daemon is started on the first incoming connection :

+

Or you can enable SSH Daemon socket so the daemon is started on the first incoming connection:

+

# systemctl enable sshd.socket

+

If you use a different port than the default 22, you have to set "ListenStream" in the unit file (/lib/systemd/system/sshd.socket) to the appropriate port.

−

{{bc|# systemctl enable sshd.socket}}

+

=== Connecting to the server ===

−

−

===Connecting to the server===

To connect to a server, run:

To connect to a server, run:

$ ssh -p port user@server-address

$ ssh -p port user@server-address

== Other SSH clients and servers ==

== Other SSH clients and servers ==

−

+

Apart from OpenSSH, there are many SSH [[Wikipedia:Comparison of SSH clients|clients]] and [[Wikipedia:Comparison of SSH servers|servers]] avaliable.

−

Apart from OpenSSH, there are many [http://en.wikipedia.org/wiki/Comparison_of_SSH_clients SSH clients] and [http://en.wikipedia.org/wiki/Comparison_of_SSH_servers servers] avaliable.

=== Dropbear ===

=== Dropbear ===

−

+

[[Wikipedia:Dropbear (software)|Dropbear]] is a SSH-2 client and server. {{AUR|dropbear}} is available in the [[AUR]].

−

[http://en.wikipedia.org/wiki/Dropbear_%28software%29 Dropbear] is ssh 2 client and server. It's only SSH2 capable (protocol 1 is considered insecure). You could install via [https://aur.archlinux.org/packages.php?ID=1657 AUR]

Remote terminal application that allows roaming, supports intermittent connectivity, and provides intelligent local echo and line editing of user keystrokes. Mosh is a replacement for SSH. It's more robust and responsive, especially over Wi-Fi, cellular, and long-distance links.

Remote terminal application that allows roaming, supports intermittent connectivity, and provides intelligent local echo and line editing of user keystrokes. Mosh is a replacement for SSH. It's more robust and responsive, especially over Wi-Fi, cellular, and long-distance links.

−

[[pacman|Install]] {{Pkg|mosh}} from the [[official repositories]] or the latest revision from {{AUR|mosh-git}}.

+

[[pacman|Install]] {{Pkg|mosh}} from the [[official repositories]] or the latest revision {{AUR|mosh-git}} in the [[AUR]].

−

If you get "mosh requires a UTF-8 locale.", see the FAQ on the website.

+

== Tips and tricks ==

=== Encrypted SOCKS tunnel ===

=== Encrypted SOCKS tunnel ===

Line 277:

Line 267:

==== Step 1: start the connection ====

==== Step 1: start the connection ====

−

You only have to execute this single command in your favorite terminal to start the connection:

+

You only have to execute this single command to start the connection:

$ ssh -ND 4711 user@host

$ ssh -ND 4711 user@host

where {{Ic|"user"}} is your username at the SSH server running at the {{Ic|"host"}}. It will ask for your password, and then you're connected! The {{Ic|"N"}} flag disables the interactive prompt, and the {{Ic|"D"}} flag specifies the local port on which to listen on (you can choose any port number if you want).

where {{Ic|"user"}} is your username at the SSH server running at the {{Ic|"host"}}. It will ask for your password, and then you're connected! The {{Ic|"N"}} flag disables the interactive prompt, and the {{Ic|"D"}} flag specifies the local port on which to listen on (you can choose any port number if you want).

Line 329:

Line 319:

Also:

Also:

* Enable the '''ForwardX11''' option in {{ic|ssh_config}} on the '''client'''.

* Enable the '''ForwardX11''' option in {{ic|ssh_config}} on the '''client'''.

−

+

* Enable the '''ForwardX11Trusted''' if gui is drawing badly.

−

Enable the '''ForwardX11Trusted''' can help when gui drawing badly.

You need to restart the ssh daemon on the server for these changes to take effect, of course.

You need to restart the ssh daemon on the server for these changes to take effect, of course.

Line 400:

Line 389:

Another way of making these changes permanent is to create an alias in {{ic|~/.bashrc}}:

Another way of making these changes permanent is to create an alias in {{ic|~/.bashrc}}:

alias ssh='ssh -C4c arcfour,blowfish-cbc'

alias ssh='ssh -C4c arcfour,blowfish-cbc'

−

−

==== Troubleshooting ====

−

Make sure your DISPLAY string is resolveable on the remote end:

−

−

$ ssh -X user@server-address

−

server $ echo $DISPLAY

−

localhost:10.0

−

server $ telnet localhost 6010

−

localhost/6010: lookup failure: Temporary failure in name resolution

−

−

can be fixed by adding localhost to {{ic|/etc/hosts}}.

=== Mounting a remote filesystem with SSHFS ===

=== Mounting a remote filesystem with SSHFS ===

−

+

Please refer to the [[Sshfs]] article to use sshfs to mount a remote system - accessible via SSH - to a local folder, so you will be able to do any operation on the mounted files with any tool (copy, rename, edit with vim, etc.). Using sshfs instead of shfs is generally preferred as a new version of shfs hasn't been released since 2004.

−

[[pacman|Install]] {{Pkg|sshfs}} from the [[official repositories]].

−

−

Load the Fuse module

−

# modprobe fuse

−

Add fuse to the ''modules'' array in {{ic|/etc/rc.conf}} to load it on each system boot.

−

−

Mount the remote folder using sshfs

−

# mkdir ~/remote_folder

−

# sshfs USER@remote_server:/tmp ~/remote_folder

−

−

The command above will cause the folder /tmp on the remote server to be mounted as ~/remote_folder on the local machine. Copying any file to this folder will result in transparent copying over the network using SFTP. Same concerns direct file editing, creating or removing.

−

−

When we’re done working with the remote filesystem, we can unmount the remote folder by issuing:

−

# fusermount -u ~/remote_folder

−

−

If we work on this folder on a daily basis, it is wise to add it to the {{ic|/etc/fstab}} table. This way is can be automatically mounted upon system boot or mounted manually (if {{Ic|noauto}} option is chosen) without the need to specify the remote location each time. Here is a sample entry in the table:

=== Automatically logout all SSH users when the sshd daemon is shutdown ===

=== Automatically logout all SSH users when the sshd daemon is shutdown ===

+

{{out of date|rc.local is deprecated with systemd}}

To automatically log out all remote ssh users when the sshd server system shuts down, for reboot or halt, add this line to /etc/rc.local.shutdown on the sshd server:

To automatically log out all remote ssh users when the sshd server system shuts down, for reboot or halt, add this line to /etc/rc.local.shutdown on the sshd server:

Line 500:

Line 462:

The session will end once you type {{ic|exit}} in the session, or the autossh process receives a SIGTERM, SIGINT of SIGKILL signal.

The session will end once you type {{ic|exit}} in the session, or the autossh process receives a SIGTERM, SIGINT of SIGKILL signal.

+

+

If you want to automatically start autossh, it is now easy to get systemd to manage this for you. For example, you could create a systemd unit file like this:

+

+

[Unit]

+

Description=AutoSSH service for port 2222

+

After=network.target

+

+

[Service]

+

ExecStart=/usr/bin/autossh -M 0 2222:localhost:2222 foo@bar.com

+

+

Then place this in, for example, /etc/systemd/system/system/autossh.service. Of course, you can make this unit more complex if necessary (see the systemd documentation for details), and obviously you can use your own options for autossh.

+

+

You can then enable your autossh tunnels with, e.g.:

+

+

$ systemctl start autossh

+

(or whatever you called the service file)

+

+

It is also easy to maintain several autossh processes, to keep several tunnels alive. Just create multiple .service files with different names.

== Troubleshooting ==

== Troubleshooting ==

Line 509:

Line 489:

Find your internal network address.

Find your internal network address.

−

ifconfig

+

ip a

Find your interface device and look for the inet field. Then access your router's configuration web interface, using your router's IP (find this on the web). Tell your router to forward it to your inet IP. Go to [http://portforward.com/] for more instructions on how to do so for your particular router.

Find your interface device and look for the inet field. Then access your router's configuration web interface, using your router's IP (find this on the web). Tell your router to forward it to your inet IP. Go to [http://portforward.com/] for more instructions on how to do so for your particular router.

==== Is SSH running and listening? ====

==== Is SSH running and listening? ====

−

# netstat -tnlp | grep ssh

+

$ ss -tnlp

−

If the above command doesn't display anything, then SSH is NOT running. Check {{ic|/var/log/messages}} for errors etc.

+

If the above command do not show SSH port is open, SSH is NOT running. Check {{ic|/var/log/messages}} for errors etc.

==== Are there firewall rules blocking the connection? ====

==== Are there firewall rules blocking the connection? ====

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{{out of date|rc.d is deprecated with systemd}}

Flush your iptables rules to make sure they are not interfering:

Flush your iptables rules to make sure they are not interfering:

Line 555:

Line 536:

===== Diagnosis via Wireshark =====

===== Diagnosis via Wireshark =====

−

First install Wireshark using pacman.

First install Wireshark using pacman.

pacman -Sy wireshark-cli

pacman -Sy wireshark-cli

Line 562:

Line 542:

tshark -f "tcp port 22" -i NET_IF

tshark -f "tcp port 22" -i NET_IF

−

where NET_IF is the network interface for a WAN connection (see {{ic|ifconfig}} to check). If you aren't receiving any packets while trying to connect remotely, you can be very sure that your ISP is blocking the incoming traffic on port 22.

+

where NET_IF is the network interface for a WAN connection (see {{ic|ip a}} to check). If you aren't receiving any packets while trying to connect remotely, you can be very sure that your ISP is blocking the incoming traffic on port 22.

===== Possible solution =====

===== Possible solution =====

Line 572:

Line 552:

Also make sure that other "Port" configuration lines in the file are commented out. Just commenting "Port 22" and putting "Port 1234" won't solve the issue because then sshd will only listen on port 1234. Use both lines to run the SSH server on both ports.

Also make sure that other "Port" configuration lines in the file are commented out. Just commenting "Port 22" and putting "Port 1234" won't solve the issue because then sshd will only listen on port 1234. Use both lines to run the SSH server on both ports.

−

Restart the server {{ic|/etc/rc.d/sshd restart}} and you're almost done. You still have to configure your client(s) to use the other port instead of the default port. There are numerous solutions to that problem, but let's cover two of them here.

+

Restart the server {{ic|systemctl restart sshd.service}} and you're almost done. You still have to configure your client(s) to use the other port instead of the default port. There are numerous solutions to that problem, but let's cover two of them here.

===== Client configuration (Quick/Greedy fix) =====

===== Client configuration (Quick/Greedy fix) =====

−

Create an alias in your {{ic|.bashrc}}. Instead of using {{ic|ssh -p 1234 user@myssh.domain.com}} every time you connect, create an alias. For example, add

Create an alias in your {{ic|.bashrc}}. Instead of using {{ic|ssh -p 1234 user@myssh.domain.com}} every time you connect, create an alias. For example, add

alias myssh='ssh -p 1234 user@myssh.domain.com'

alias myssh='ssh -p 1234 user@myssh.domain.com'

Line 581:

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===== Client configuration 2 (Recommended/Systematic approach) =====

===== Client configuration 2 (Recommended/Systematic approach) =====

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Create or use {{ic|~/.ssh/config}} to solve the issue. Add the following lines:

Create or use {{ic|~/.ssh/config}} to solve the issue. Add the following lines:

One possible cause for this is the need of certain SSH clients to find an absolute path (one returned by {{Ic|whereis -b [your shell]}}, for instance) in {{Ic|$SHELL}}, even if the shell's binary is located in one of the {{Ic|$PATH}} entries. Another reason can be that the user is no member of the ''network'' group.

−

One possible cause for this is the need of certain SSH clients to find an absolute path (one returned by {{Ic|whereis -b [your shell]}}, for instance) in {{Ic|$SHELL}}, even if the shell's binary is located in one of the {{Ic|$PATH}} entries. Another reason can be that the user is no member of the ''network'' group.

+

==="Terminal unknown" or "Error opening terminal" error message===

+

With ssh it is possible to receive errors like "Terminal unknown" upon logging in. Starting ncurses applications like nano fails with the message "Error opening terminal". There are two methods to this problem, a quick one using the $TERM variable and a profound one using the terminfo file.

+

+

====Workaround by setting the $TERM variable====

+

After connecting to the remote server set the $TERM variable to "xterm" with the following command.

+

+

{{ic|TERM&#61;xterm}}

+

+

This method is a workaround and should be used on ssh servers you do seldomly connect to, because it can have unwanted side effects. Also you have to repeat the command after every connection, or alternatively set it in ~.bashrc .

+

====Solution using terminfo file====

+

A profound solution is transferring the terminfo file of the terminal on your client computer to the ssh server. In this example we cover how to setup the terminfo file for the "rxvt-unicode-256color" terminal.

+

Create the directory containing the terminfo files on the ssh server, while you are logged in to the server issue this command:

+

+

{{ic| mkdir -p ~/.terminfo/r/}}

+

+

Now copy the terminfo file of your terminal to the new directory. Replace ''"rxvt-unicode-256color"'' with your client's terminal in the following command and ''ssh-server'' with the relevant user and server adress.

After logging in and out from the ssh server the problem should be fixed.

== See also ==

== See also ==

Revision as of 14:52, 4 March 2013

zh-CN:Secure ShellSecure Shell (SSH) is a network protocol that allows data to be exchanged over a secure channel between two computers. Encryption provides confidentiality and integrity of data. SSH uses public-key cryptography to authenticate the remote computer and allow the remote computer to authenticate the user, if necessary.

SSH is typically used to log into a remote machine and execute commands, but it also supports tunneling, forwarding arbitrary TCP ports and X11 connections; file transfer can be accomplished using the associated SFTP or SCP protocols.

An SSH server, by default, listens on the standard TCP port 22. An SSH client program is typically used for establishing connections to an sshd daemon accepting remote connections. Both are commonly present on most modern operating systems, including Mac OS X, GNU/Linux, Solaris and OpenVMS. Proprietary, freeware and open source versions of various levels of complexity and completeness exist.

OpenSSH

OpenSSH (OpenBSD Secure Shell) is a set of computer programs providing encrypted communication sessions over a computer network using the ssh protocol. It was created as an open source alternative to the proprietary Secure Shell software suite offered by SSH Communications Security. OpenSSH is developed as part of the OpenBSD project, which is led by Theo de Raadt.

OpenSSH is occasionally confused with the similarly-named OpenSSL; however, the projects have different purposes and are developed by different teams, the similar name is drawn only from similar goals.

Even though the port ssh is running on could be detected by using a port-scanner like nmap, changing it will reduce the number of log entries caused by automated authentication attempts. To help select a port review the list of TCP and UDP port numbers.

Managing the sshd daemon

You can start the sshd daemon with the following command:

# systemctl start sshd

You can enable the sshd daemon at startup with the following command:

# systemctl enable sshd.service

Warning: Systemd is an asynchronous starting process. If you bind the SSH daemon to a specific IP address ListenAddress 192.168.1.100 it may fail to load during boot since the default sshd.service unit file has no dependency on network interfaces being enabled. When binding to an IP address, you will need to add After=network.target to a custom sshd.service unit file. See Systemd#Replacing provided unit files.

Or you can enable SSH Daemon socket so the daemon is started on the first incoming connection:

# systemctl enable sshd.socket

If you use a different port than the default 22, you have to set "ListenStream" in the unit file (/lib/systemd/system/sshd.socket) to the appropriate port.

SSH alternative: Mobile Shell - responsive, survives disconnects

Remote terminal application that allows roaming, supports intermittent connectivity, and provides intelligent local echo and line editing of user keystrokes. Mosh is a replacement for SSH. It's more robust and responsive, especially over Wi-Fi, cellular, and long-distance links.

Tips and tricks

Encrypted SOCKS tunnel

This is highly useful for laptop users connected to various unsafe wireless connections. The only thing you need is an SSH server running at a somewhat secure location, like your home or at work. It might be useful to use a dynamic DNS service like DynDNS so you do not have to remember your IP-address.

Step 1: start the connection

You only have to execute this single command to start the connection:

$ ssh -ND 4711 user@host

where "user" is your username at the SSH server running at the "host". It will ask for your password, and then you're connected! The "N" flag disables the interactive prompt, and the "D" flag specifies the local port on which to listen on (you can choose any port number if you want).

One way to make this easier is to put an alias line in your ~/.bashrc file as following:

alias sshtunnel="ssh -ND 4711 -v user@host"

It's nice to add the verbose "-v" flag, because then you can verify that it's actually connected from that output. Now you just have to execute the "sshtunnel" command :)

Step 2: configure your browser (or other programs)

The above step is completely useless if you do not configure your web browser (or other programs) to use this newly created socks tunnel. Since the current version of SSH supports both SOCKS4 and SOCKS5, you can use either of them.

X11 forwarding

To run graphical programs through a SSH connection you can enable X11 forwarding. An option needs to be set in the configuration files on the server and client (here "client" means your (desktop) machine your X11 Server runs on, and you will run X applications on the "server").

You need to restart the ssh daemon on the server for these changes to take effect, of course.

To use the forwarding, log on to your server through ssh:

$ ssh -X -p port user@server-address

If you receive errors trying to run graphical applications try trusted forwarding instead:

$ ssh -Y -p port user@server-address

You can now start any X program on the remote server, the output will be forwarded to your local session:

$ xclock

If you get "Cannot open display" errors try the following command as the non root user:

$ xhost +

the above command will allow anybody to forward X11 applications. To restrict forwarding to a particular host type:

$ xhost +hostname

where hostname is the name of the particular host you want to forward to. Type "man xhost" for more details.

Be careful with some applications as they check for a running instance on the local machine. Firefox is an example. Either close running Firefox or use the following start parameter to start a remote instance on the local machine

$ firefox -no-remote

If you get "X11 forwarding request failed on channel 0" when you connect (and the server /var/log/errors.log shows "Failed to allocate internet-domain X11 display socket"), try to either

Enable the AddressFamily any option in sshd_config on the server, or

Set the AddressFamily option in sshd_config on the server to inet.

Setting it to inet may fix problems with Ubuntu clients on IPv4.

Forwarding other ports

In addition to SSH's built-in support for X11, it can also be used to securely tunnel any TCP connection, by use of local forwarding or remote forwarding.

Local forwarding opens a port on the local machine, connections to which will be forwarded to the remote host and from there on to a given destination. Very often, the forwarding destination will be the same as the remote host, thus providing a secure shell and, e.g. a secure VNC connection, to the same machine. Local forwarding is accomplished by means of the -L switch and it's accompanying forwarding specification in the form of <tunnel port>:<destination address>:<destination port>.

Thus:

$ ssh -L 1000:mail.google.com:25 192.168.0.100

will use SSH to login to and open a shell on 192.168.0.100, and will also create a tunnel from the local machine's TCP port 1000 to mail.google.com on port 25. Once established, connections to localhost:1000 will connect to the Gmail SMTP port. To Google, it will appear that any such connection (though not necessarily the data conveyed over the connection) originated from 192.168.0.100, and such data will be secure as between the local machine and 192.168.0.100, but not between 192.168.0.100, unless other measures are taken.

Similarly:

$ ssh -L 2000:192.168.0.100:6001 192.168.0.100

will allow connections to localhost:2000 which will be transparently sent to the remote host on port 6001. The preceding example is useful for VNC connections using the vncserver utility--part of the tightvnc package--which, though very useful, is explicit about its lack of security.

Remote forwarding allows the remote host to connect to an arbitrary host via the SSH tunnel and the local machine, providing a functional reversal of local forwarding, and is useful for situations where, e.g., the remote host has limited connectivity due to firewalling. It is enabled with the -R switch and a forwarding specification in the form of <tunnel port>:<destination address>:<destination port>.

Thus:

$ ssh -R 3000:irc.freenode.net:6667 192.168.0.200

will bring up a shell on 192.168.0.200, and connections from 192.168.0.200 to itself on port 3000 (remotely speaking, localhost:3000) will be sent over the tunnel to the local machine and then on to irc.freenode.net on port 6667, thus, in this example, allowing the use of IRC programs on the remote host to be used, even if port 6667 would normally be blocked to it.

Both local and remote forwarding can be used to provide a secure "gateway," allowing other computers to take advantage of an SSH tunnel, without actually running SSH or the SSH daemon by providing a bind-address for the start of the tunnel as part of the forwarding specification, e.g. <tunnel address>:<tunnel port>:<destination address>:<destination port>. The <tunnel address> can be any address on the machine at the start of the tunnel, localhost, * (or blank), which, respectively, allow connections via the given address, via the loopback interface, or via any interface. By default, forwarding is limited to connections from the machine at the "beginning" of the tunnel, i.e. the <tunnel address> is set to localhost. Local forwarding requires no additional configuration, however remote forwarding is limited by the remote server's SSH daemon configuration. See the GatewayPorts option in sshd_config(5) for more information.

Speeding up SSH

You can make all sessions to the same host use a single connection, which will greatly speed up subsequent logins, by adding these lines under the proper host in /etc/ssh/ssh_config:

ControlMaster auto
ControlPath ~/.ssh/socket-%r@%h:%p

Changing the ciphers used by SSH to less cpu-demanding ones can improve speed. In this aspect, the best choices are arcfour and blowfish-cbc. Please do not do this unless you know what you are doing; arcfour has a number of known weaknesses. To use them, run SSH with the "c" flag, like this:

$ ssh -c arcfour,blowfish-cbc user@server-address

To use them permanently, add this line under the proper host in /etc/ssh/ssh_config:

Ciphers arcfour,blowfish-cbc

Another option to improve speed is to enable compression with the "C" flag. A permanent solution is to add this line under the proper host in /etc/ssh/ssh_config:

Compression yes

Login time can be shorten by using the "4" flag, which bypasses IPv6 lookup. This can be made permanent by adding this line under the proper host in /etc/ssh/ssh_config:

AddressFamily inet

Another way of making these changes permanent is to create an alias in ~/.bashrc:

alias ssh='ssh -C4c arcfour,blowfish-cbc'

Mounting a remote filesystem with SSHFS

Please refer to the Sshfs article to use sshfs to mount a remote system - accessible via SSH - to a local folder, so you will be able to do any operation on the mounted files with any tool (copy, rename, edit with vim, etc.). Using sshfs instead of shfs is generally preferred as a new version of shfs hasn't been released since 2004.

Keep alive

Your ssh session will automatically log out if it is idle. To keep the connection active (alive) add this to ~/.ssh/config or to /etc/ssh/ssh_config on the client.

ServerAliveInterval 120

This will send a "keep alive" signal to the server every 120 seconds.

Conversely, to keep incoming connections alive, you can set

ClientAliveInterval 120

(or some other number greater than 0) in /etc/ssh/sshd_config on the server.

Saving connection data in ssh config

Whenever you want to connect to a ssh server, you usually have to type at least its address and the username. To save that typing work for servers you regularly connect to, you can use the personal $HOME/.ssh/config or the global /etc/ssh/ssh_config files as shown in the following example:

Now you can simply connect to the server by using the name you specified:

$ ssh myserver

To see a complete list of the possible options, check out ssh_config's manpage on your system or the ssh_config documentation on the official website.

Changing the Bash prompt when logged in over SSH

It can sometimes be useful to make a difference between your local and your remote prompt, in particular when they are both configured in the same way. To do that, just insert this in the remote server's bashrc file:

Automatically logout all SSH users when the sshd daemon is shutdown

To automatically log out all remote ssh users when the sshd server system shuts down, for reboot or halt, add this line to /etc/rc.local.shutdown on the sshd server:

who | cut -d " " -f1 | uniq | xargs pkill -KILL -u

This prevents ssh client terminals from hanging during a lengthy timeout, which eventually ends with:

Write failed: Broken pipe

Autossh - automatically restarts SSH sessions and tunnels

When a ssh session or tunnel cannot be kept alive, because for example bad network conditions cause the sshd client to disconnect, you can use Autossh to automatically restart them. Autossh can be installed from the official repositories.

Then place this in, for example, /etc/systemd/system/system/autossh.service. Of course, you can make this unit more complex if necessary (see the systemd documentation for details), and obviously you can use your own options for autossh.

You can then enable your autossh tunnels with, e.g.:

$ systemctl start autossh

(or whatever you called the service file)

It is also easy to maintain several autossh processes, to keep several tunnels alive. Just create multiple .service files with different names.

Troubleshooting

Connection refused or timeout problem

Is your router doing port forwarding?

The first thing is to make sure that your router knows to forward any incoming ssh connection to your machine. Your external IP is given to you by your ISP, and it is associated with any requests coming out of your router. So your router needs to know that any incoming ssh connection to your external IP needs to be forwarded to your machine running sshd.

Find your internal network address.

ip a

Find your interface device and look for the inet field. Then access your router's configuration web interface, using your router's IP (find this on the web). Tell your router to forward it to your inet IP. Go to [1] for more instructions on how to do so for your particular router.

Is SSH running and listening?

$ ss -tnlp

If the above command do not show SSH port is open, SSH is NOT running. Check /var/log/messages for errors etc.

Is the traffic even getting to your computer?

Start a traffic dump on the computer you're having problems with:

# tcpdump -lnn -i any port ssh and tcp-syn

This should show some basic information, then wait for any matching traffic to happen before displaying it. Try your connection now. If you do not see any output when you attempt to connect, then something outside of your computer is blocking the traffic (e. g., hardware firewall, NAT router etc.).

Your ISP or a third party blocking default port?

Note: Try this step if you KNOW you aren't running any firewalls and you know you have configured the router for DMZ or have forwarded the port to your computer and it still doesn't work. Here you will find diagnostic steps and a possible solution.

That means that something is rejecting your TCP traffic on port 22. Basically that port is stealth, either by your firewall or 3rd party intervetion (like an ISP blocking and/or rejecting incoming traffic on port 22). If you know you aren't running any firewall on your computer, and you know that Gremlins aren't growing in your routers and switches, then your ISP is blocking the traffic.

To double check, you can run Wireshark on your server and listen to traffic on port 22. Since Wireshark is a Layer 2 Packet Sniffing utility, and TCP/UDP are Layer 3 and above (See IP Network stack), if you don't receive anything while connecting remotely, a third party is most likely to be blocking the traffic on that port to your server.

Diagnosis via Wireshark

First install Wireshark using pacman.

pacman -Sy wireshark-cli

And then run it using,

tshark -f "tcp port 22" -i NET_IF

where NET_IF is the network interface for a WAN connection (see ip a to check). If you aren't receiving any packets while trying to connect remotely, you can be very sure that your ISP is blocking the incoming traffic on port 22.

Possible solution

The solution is just to use some other port that the ISP isn't blocking. Open the /etc/ssh/sshd_config and configure the file to use different ports. For example, add:

Port 22
Port 1234

Also make sure that other "Port" configuration lines in the file are commented out. Just commenting "Port 22" and putting "Port 1234" won't solve the issue because then sshd will only listen on port 1234. Use both lines to run the SSH server on both ports.

Restart the server systemctl restart sshd.service and you're almost done. You still have to configure your client(s) to use the other port instead of the default port. There are numerous solutions to that problem, but let's cover two of them here.

Client configuration (Quick/Greedy fix)

Create an alias in your .bashrc. Instead of using ssh -p 1234 user@myssh.domain.com every time you connect, create an alias. For example, add

alias myssh='ssh -p 1234 user@myssh.domain.com'

to your .bashrc and restart bash. Just type myssh and you'll connect !

Client configuration 2 (Recommended/Systematic approach)

Create or use ~/.ssh/config to solve the issue. Add the following lines:

Host myssh.domain.com
Port 1234

And ssh will automatically use port 1234 on connecting without you excplicitly having to define the port while connecting.

Read from socket failed: connection reset by peer

Recent versions of openssh sometimes fail with the above error message, due to a bug involving elliptic curve cryptography. In that case, edit the file

"[your shell]: No such file or directory" / ssh_exchange_identification problem

One possible cause for this is the need of certain SSH clients to find an absolute path (one returned by whereis -b [your shell], for instance) in $SHELL, even if the shell's binary is located in one of the $PATH entries. Another reason can be that the user is no member of the network group.

"Terminal unknown" or "Error opening terminal" error message

With ssh it is possible to receive errors like "Terminal unknown" upon logging in. Starting ncurses applications like nano fails with the message "Error opening terminal". There are two methods to this problem, a quick one using the $TERM variable and a profound one using the terminfo file.

Workaround by setting the $TERM variable

After connecting to the remote server set the $TERM variable to "xterm" with the following command.

TERM=xterm

This method is a workaround and should be used on ssh servers you do seldomly connect to, because it can have unwanted side effects. Also you have to repeat the command after every connection, or alternatively set it in ~.bashrc .

Solution using terminfo file

A profound solution is transferring the terminfo file of the terminal on your client computer to the ssh server. In this example we cover how to setup the terminfo file for the "rxvt-unicode-256color" terminal.
Create the directory containing the terminfo files on the ssh server, while you are logged in to the server issue this command:

mkdir -p ~/.terminfo/r/

Now copy the terminfo file of your terminal to the new directory. Replace "rxvt-unicode-256color" with your client's terminal in the following command and ssh-server with the relevant user and server adress.